1. Field of the Invention
Many prior-art processes have been developed for the preparation of urea and its derivatives using high temperature/high pressure processes and various feedstock chemicals, such as CO.sub.2 NH.sub.3, etc.
2. Description of the Prior Art
German Offen. No. 2,809,858 describes a process for reacting nitric oxide or nitrous oxide with carbon monoxide and hydrogen at 200.degree. C. to 600.degree. C. over Pd, Pb, Rh, or Monel catalysts to give ammonium cyanate. This process requires further conversion of the ammonium cyanate to urea and high energy requirements for the initial process. The process is generally carried out in dilute streams of helium, an expensive and valuable rare gas.
In a 1978 article, R. J. H. Voorhoeve and coworkers [Science 200, 759-761 (1978)]reacted carbon monoxide, nitric oxide, water, and hydrogen in a diluted stream of helium over ruthenium, rhodium, palladium, platinum, and iridium catalysts dispersed on alumina at 240.degree. C. to 500.degree. C. to give principally ammonium cyanate. Typically, the reactant gases (CO, NO, H.sub.2 O, H.sub.2) comprised only 11.5% of the total gas flow, and of this only 17% was converted to ammonium cyanate. Further work by these same authors [J. Catal. 53 (2), 251-259 (1978)]indicates a maximum selectivity of 98% at 360.degree. C. to 400.degree. C. for ammonium cyanate from NO, CO, H.sub.2 diluted in helium over Pt, Cu-Ni, Os, and ruthenium metal catalysts.
R. J. H. Voorhoeve, [J. Catal. 54 (1), 102-105 (1978)]also has reported the reduction of No over platinum catalysts at 650.degree. C. to 800.degree. C. to give principally nitrogen with some ammonia, hydrogen cyanide, and ammonium cyanate. Subsequent reports [J. Catal. 54 (2), 268-280 (1978)]indicate that the major products of the reaction of CO, NO, and H.sub.2 over unsupported Pd, Ir, and Pt-10% Rh metal catalysts are ammonium cyanate and HNCO (isocyanic acid). The best yield of HNCO was 75% over iridium metal catalyst.
Cyanates (Belgium Pat. No. 876,483, Sept. 17, 1979) also have been produced by the catalytic hydrogenation of a mixture of nitric oxide and carbon monoxide in the presence of ammonia and a nobel metal, such as palladium, iridium, or mixtures of these. The formation of ammonium cyanate and isocyanic acid (HNCO) by a low-pressure catalytic process is described by Trimble et al in U.S. Pat. No. 4,174,377. In the reaction, a mixture of 0.3% NO, 5% CO, and 0.5% H.sub.2 (with the balance being helium) is passed over a Pd catalyst at 550.degree. C. and 40 L/h m.sup.2. Conversion to cyanate compounds was 72.5%. All of the above processes emphasize the formation of cyanate rather than urea. Also, all are carried out at high temperatures and generally as dilute gas streams in helium, an expensive and rare gas. The production of urea would require further reaction of the ammonium cyanate.
Many important commercial applications have been developed for the urea produced from the present invention, for example, as in fertilizer preparation (both solid and liquid) and as resin monomers.
The process of the present invention provides a method for carrying out the reaction of nitric oxide, carbon monoxide, and a hydrogen source (such as methanol, hydrogen, or water) over palladium or rhodium noble metal catalysts or an inert support (such as alumina or silica) at low temperatures and pressures to directly produce urea and/or its derivatives.